1. Field of the Invention
The present invention relates to an image forming system that can print images on both sides of a sheet (duplex printing) by connecting plural image forming apparatuses in series.
2. Description of Related Art
An image forming system connecting plural image forming apparatuses in series has been used in recent years in order to improve productivity when the duplex printing is performed. For example, a system configuration has been known in which two image forming apparatuses are connected in series through a sheet-reversing apparatus. In such a system, an image forming apparatus at an upstream side forms an image on a top surface of the sheet, heats and presses the sheet to fix the formed image on the top surface of the sheet and transports the sheet to a sheet-reversing apparatus. The sheet-reversing apparatus then reverses a surface of the sheet and transports the reversed sheet to an image forming apparatus at a downstream side. The image forming apparatus at the downstream side forms an image on a back surface of the sheet, fixes the formed image on the back surface of the sheet and ejects the fixed sheet to a paper ejection tray. Because such an image forming system forms the images on the top and back surfaces of the sheet by separate image forming apparatuses, a printing speed is improved as compared with a case where one image forming apparatus carries out the duplex printing.
For example, Japanese Patent Application Publications Nos. 2007-137012 and 2009-300703 have disclosed technologies of such an image forming system. Japanese Patent Application Publication No. 2007-137012 has disclosed a printing system in which a plurality of print engines are arranged and by performing an inspection mode where each printing engine prints a predetermined inspection chart on the same surface of the sheet, a determination can be made whether color reproducibility coincides between both print engines or not. Japanese Patent Application Publication No. 2009-300703 has disclosed a printing system in which a plurality of printing devices are connected in series to perform duplex printing and by measuring the images formed on the top surface of the printing sheet and the back surface thereof by sensors, a feedback control for maintaining the color reproducibility in the printing system optimal is performed on the basis of the measured result thereof.
In any of the technologies disclosed in Japanese Patent Application Publications Nos. 2007-137012 and 2009-300703, however, the sheet passed through the image forming apparatus at an upstream side is heated under the fixing step, so that the image forming apparatus at a downstream side receives the heated sheet. Accordingly, in the image forming apparatus at the downstream side, some issues based on temperature rise in the apparatus occur. For example, in the image forming apparatus at the downstream side, any deterioration occurs in materials such as toner or developing materials.